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As of version 3.15 of Dirac Live, the previously hard-coded 120 Hz low-pass filter (LPF) embedded in the Dirac Live filter design for the low-frequency effects (LFE) channel in Bass Control and Active Room Treatment (i.e. not in the Dirac Live Room Correction filter) has been made optional.

The low-pass filter will be enabled by default, so a user that doesn't actively disable the low-pass filter will have it applied. The main reason for making the LPF optional is that it is not always desired by end-users, as they would like to hear the full content of the LFE channel.

The LPF was introduced upon request from device OEMs, in connection to the introduction of Dirac Live Bass Control, as part of Dirac taking over responsibility for the bass management functionality for that use case.

For consistency, Dirac also added the same LPF to the ART filter design when that was introduced. The motivation for having an LPF in the first place was to ensure that occasional high-frequency (above the currently stipulated 120 Hz LFE low-pass limit) noise, present on some Dolby TrueHD and DTS HD encoded LFE source material, doesn't propagate to the speakers. The corresponding decoders do not apply any LPF on the LFE channel, so devices are left with the option to do so instead.

Following the release of Dirac Live 3.15, users will be able to opt-out of the previously hard-coded LPF in the LFE channel. Note, however, that in Bass Control, the low-pass filter cannot be fully disabled. Opting out will only raise the cutoff frequency from 120 Hz to 250 Hz, the maximum allowed cutoff in the BC algorithm.

The setting will be checked for every new filter design using BC and ART. A user may calculate one Dirac Live filter with the LPF and one without it and export both to a device. Dirac Live will attempt to store the latest setting to be persistent for future sessions, so that a user that has opted out (or opted in) of the LFE LPF once, should not need to touch this setting again until they wish to change it. This means that when loading a project that was either created before this option was available, or, if it was set differently when originally saving the project, Dirac Live will apply the current setting and re-calculate the filters.

Known issue: LFE channel latency

The LFE low-pass filter introduces additional latency on the LFE channel of up to 5 ms. This latency is not included in the filter design and is therefore not accounted for or compensated in the overall system timing. Disabling the low-pass filter removes this latency entirely for ART designs. For BC the filter cannot be fully disabled; "disabling" it only raises the cutoff from 120 Hz to 250 Hz, meaning latency is significantly reduced but not completely removed.